The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Typically, automotive vehicles including cars and trucks have an internal combustion engine which is coupled to at least a transmission and a differential for providing power to the drive wheels of the vehicle. An engine exhaust system which typically includes an exhaust pipe, a catalytic converter and a muffler is attached to the engine to quiet the combustion process, to clean the exhaust gases and to route the products of combustion away from the engine to a desired position typically at the rear of the vehicle. The exhaust system is supported by exhaust mounts which are positioned between the exhaust system and the frame or some other supporting structure of the vehicle body. In order to prevent engine vibrations from being transmitted to the car body, the exhaust mounts incorporate flexible members or elastic suspension members to isolate the vehicle's exhaust system from the vehicle's body. In order to effectively isolate the vehicle's exhaust system from the vehicle's body, it is preferred that the isolator include a soft on-center rate of deflection.
The common prior art exhaust mounts or isolators have included two-hole pendulum rubber isolators which include a solid rubber component or a puck that is at least three-quarters of an inch thick and which is provided with at least one pair of apertures extending therethrough. The apertures each receive an elongated metal stud hanger. The metal stud hanger is often provided with an enlarged tapered head that can be forced through the aperture in the isolator, but which cannot be readily removed from the isolator. The opposite end of the hanger is welded to or otherwise secured to either a support point in the vehicle or to one of the components of the exhaust system.
Other designs for isolators include elastomeric moldings having a 1-hole spoke design wherein spokes are loaded in tension and compression, elastomeric moldings having a 1-hole shear leg design that include a pair of molded legs subjected to shearing in the primary loading direction, and elastomeric moldings having a bell-shaped design.
Most high temp elastomers utilized in exhaust isolator assemblies exhibit poor tensile fatigue properties stemming from low tear strength properties. It is therefore preferred to load the elastomeric material in compression or shear. For example, the puck design, as discussed above, provides for two pins to be inserted at opposite ends of the elastomeric element which allows loading in tension on the elastomer cords connecting both ends. While this is typically the lowest cost design, it is also the most abusive to the material. In order to offset the failure risk, flexible and/or rigid bands are typically designed inside or around the outside of the elastomeric puck.
Spoke design isolators load the elastomeric material in compression and tension. The tensile loading makes the design vulnerable to fractures in overloaded conditions. The stress magnitude is directly proportional to the load divided by the minimum spoke cross-sectional area. An additional requirement of the spoke design is that the mating component or hanger pin be centered within the deflection zone while statically preloaded by the weight of the exhaust. If it is not, the voids designed into the isolator will be bottomed out or positioned in a groundout condition. This results in the soft on-center rate not being employed, thus defeating the purpose of the isolator.
Shear leg design isolators have a primary loading direction which is typically vertical and a secondary loading direction which is typically lateral. When the shear leg design is loaded in its primary loading direction, the loading method is the preferred shear style loading. Shear style loading is able to be designed desirably soft. However, the secondary loading direction inflicts tensile compressive stresses which are unfavorable for durability. In addition, the secondary loading direction has a rate that is two to three times stiffer than the primary rate which is also an unfavorable condition.
The continued development of elastomeric mounts has been directed to elastomeric mounts which include a soft on-center rate while avoiding the undesirable tension loading of the elastomeric bushing and which avoid the vulnerable stress concentrations. While this has been achieved in the prior art shear-hub designs, stress concentrations at the ends of the voids continues to be an issue.